Method and composition for electroplating cadmium

ABSTRACT

AN AQUEOUS BATH FOR PRODUCING DUCTILE, ADHERENT CADMIUM ELECTRODEPOSITS UPON METALLIC SURFACES CONTAINS CADMIUM ION, AMMONIUM CHLORIDE AND AN AMINOPOLYACETIC ACID CHELATING AGENT. THE BATH IS HIGHLY CONDUCTIVE AND OPERABLE AT LOW VOLTAGES, AND HAS A PH WHICH IS ACIDIC OR SLIGHTLY ALKALINE.

United States Patent O 3,558,447 METHOD AND COMPOSITION FOR ELECTROPLATING CADMIUM Tibor Joachim, Vernon, and Merton M. Beckwith, Tolland, Conn., assignors to Conversion Chemical Corporation, Rockville, Conn., a corporation of Connecticut N Drawing. Filed Sept. 4, 1968, Ser. No. 757,504

Int. Cl. C23b 5/10, 5/12 US. Cl. 204-50 9 Claims ABSTRACT OF THE DISCLOSURE An aqueous bath for producing ductile, adherent cadmium electrodeposits upon metallic surfaces contains cadmium ion, ammonium chloride and an aminopolyacetic acid chelating agent. The bath is highly conductive and operable at low voltages, and has a pH which is acidic or slightly alkaline.

BACKGROUND OF THE INVENTION A variety of baths have been widely employed for electroplating cadmium onto metallic substrates, and most commonly sulfates and cyanides are utilized as the primary electrolytes therein. Generally, the cyanide baths have proven quite effective and satisfactory despite certain significant objectionable features, such as high toxicity, difliculty of disposal, low current efliciency and hydrogen embrittlement of certain steels. The sulfate baths overcome many of the objectionable features of the cyanide baths, but normally they exhibit relatively low throwing power, poor efficiency at low current densities and secondary anode corrosion which results in an increase of metal in the bath.

Accordingly, it is an object of the present invention to provide a novel chloride plating bath for producing adherent cadmium deposits on various metals.

It is also .an object to provide such a bath which is operable with high current efiiciency over a fairly wide range of current density and which affords excellent throwing power and operability at low voltages.

Another object is to provide such a bath which" is effective in depositing cadmium upon cast-and malleable irons and which virtually eliminates or significantly reduces the problem of hydrogen embrittlement commonly encountered in electroplating certain ferrous alloys.

A further object is to provide a method for electroplating cadmium onto metallic workpieces utilizing such an improved chloride plating bath so as to obtain highly advantageous adherent and ductile cadmium deposits at relatively low cost and adapted to avoid substantially the diffusion of hydrogen into the ferrous alloys.

SUMMARY on THE INVENTION It has now been found that the foregoing and related objects can be readily attained .in an aqueous bath containing a 0.15 to 1.1 molar concentration of cadmium ion, a 1.0 to 4.0 molar concentration of ammonium chloride and, per mole of cadmium ion, about 0.8 to 1.6 moles of an aminopolyacetic acid chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA) and mixtures thereof. The bath is operable at a pH of about 3.7 to 7.5 and at cur rent densities ranging from less than about 5.0 to 70.0 amperes per square foot and higher.

3,558,447 Patented Jan. 26, 1971 DESCRIPTION OF THE PREFERRED EMBODIMENTS The ions which are essential for the proper operation of the baths of the present invention are cadmium, chloride and ammonium; in addition, the baths must contain an effective amount of the chelating agent. In order to avoid the presence of unnecessary and possibly interfering ions in the baths, the bath may be desirably formed by the admixture of cadmium chloride, ammonium chloride and the chelating agent. However, cadmium oxide may furnish the cadmium ion without detriment, and its use is preferred from an economic standpoint. Other ammonium compounds may supplement the ammonium chloride; for example, ammonium hydroxide may be added when adjustment of the pH to a higher value is necessary. In the event that the bath is more basic than desired, the most suitable reagent for lowering the pH is hydrochloric acid.

As has been pointed out, the baths may contain about (.15 to 1.1 molar concentration of cadmium ion; preferably the concentration of the cadmium ion is about 0.25 to 0.9 molar, and most desirably about 0.3 to 0.7. A cadmium ion concentration higher than about 1.1 molar tends to produce a deposit which is coarse and grainy, whereas the current densities attainable in baths containing less than about 0.15 mole per liter thereof tend to be too low and result in non-uniform coatings which exhibit evidence of burning in some areas.

Although the ammonium chloride concentration should be maintained at about 1.0 to 4.0 moles per liter, the baths preferably contain about 1.5 to 3.5 and, most desirably, about 2.0 to 3.0 moles per liter. It has been found that use of less than a 1.0 molar concentration of this salt produces a poorly conductive bath which results in coatings of marginal quality. Raising the concentration thereof beyond about 4.0 molar produces little, if any, additional increase in current density, and at the same time it tends to cause a very undesirable sludge to build up and coat the anode surface.

The concentration of the arninopolyacetic acid chelating agent which is necessary for the proper operation of the present baths is directly proportional to the concentration of cadmium ion therein, and it has been found that a molar ratio of the chelating agent to cadmium ion in the range of 0:8 to l.'6:1.0 is required. For optimum results, the ratio should be maintained at about 1.0 to 1.25 z 1.0. Satisfactory operation of the bath and acceptable deposits of cadmium depend upon the adequate chelation of the cadmium ion, so that at least 0.8 mole of chelating agent must be present per mole of cadmium; a lower ratio tends to produce deposits which are spongy, coarse, and porous, particularly at the extremes of current density. On the other hand, if the amount of chelating agent is too large the plating of cadmium is inhibited to an unsatisfactory extent and there is a strong tendency for gas evolution to occur at the cathode.

In addition to the foregoing components the baths of the present invention may also contain conventional surfactants, brighteners, chelating agents, buffers or other additives which are commonly included in baths of this type. However, it is particularly notable and an advantageous aspect of the present baths that they do not require any such additional agents for satisfactory operation and may, in fact, produce optimum results when such other agents are excluded. Since the essential components of the baths are normally solid materials, the bath may conveniently be prepared from dry formulations containing the components in the proportions necessary 3 to produce the desired concentrations upon dissolution in water. A specific dry powder formulation which has proven useful is as follows:

This powder is added to water in the amount of about 350 to 450 grams per liter and will normally require about 120 to 200 milliliters per liter of ammonium hydroxide to provide a pH in the preferred operating range of about 4.5 to 5.5.

For effective operation, it is important that the temperature of the bath be maintained at about 25 to 65 centigrade and that the pH be about 3.5 to 7.5. For optimum results, a temperature range of about 32 to 52 centigrade and a pH of about 4.5 to 5.5 are preferred. If the concentrations of the various constituents indicated hereinbefore are maintained within the ranges specified, the baths are operable with high plating efficiencies at low voltages, i.e., at voltages just above the deposition potential for cadmium. However, the voltage is preferably adjusted to provide a current density of about 5.0 to 70.0 amperes per square foot, and desirably, the baths are operated with current densities of about 20.0 to 55.0 amperes per square foot. The baths of the present invention exhibit excellent throwing power over a wide range of current density and allow efficient plating under a variety of conditions.

Particularly advantageous is the fact that the baths can be used to plate cadmium upon certain steel alloys for which many of the prior art baths are unsuitable due to the levels of hydrogen diffusion and embrittlement which are caused thereby. Moreover, although some prior art baths may be suitable for plating such alloys, the coatings produced are frequently of a poor quality and, as far as is known, the degree of embrittlement produced thereby is invariably dependent upon the conditions of operation and particularly the current density applied. More specifically, as the current density is decreased, the known prior art baths tend to produce higher levels of embrittlement. The baths of the present invention, on the other hand, have been found to produce consistently good deposits and low levels of embrittlement, relatively independently of the current density used, and this is a most surprising aspect of the invention disclosed.

Although cadimum alloy anodes may be employed, highly refined anodes of pure cadmium are most desirable, both from the standpoint of avoiding corrosion problems and also from the standpoint of avoiding the introduction of extraneous ions which would tend to interfere with optimum operation. Insoluble anodes are also undersirable because their use with these baths is normally accompanied by the generation of chlorine and/or other gases.

Agitation encourages the production of high quality, uniform deposits and helps to avoid the development of anode sludge and film. Not only is agitation of the bath itself desirable, therefore, but the anode may also be agitated to reduce any tendency for sludge to accumulate thereat. Furthermore, agitation of the cathode (workpiece) may be desirable to obtain a uniform plate and to enable extension of the range of satisfactory current densities to higher levels.

The process is adapted to use in both still and barrel plating apparatus. Filtration is not essential but is normally beneficial, particularly when contamination of the bath is encountered from airborne impurities and by carry-over from other operations. For this purpose, various filter media including fabrics, porous stoneware and other conventional materials may be utilized. The

need for correction for the depletion of the various components of the bath is best determined by a periodic quantative analysis for the several components based upon the size of the bath and the plating rate.

Any metallic substrate which can be plated with cadmium using prior art baths may be coated in accordance with the present invention. As has been pointed out hereinbefore, the steel alloys which are susceptible to hydrogen embrittlement in prior art baths are particularly well suited to plating with the baths of the present invention because of the low levels of hydrogen diffusion and embrittlement which are produced thereby.

Illustrative of the efficacy of the present invention are the following specific examples wherein all parts are parts by weight unless otherwise indicated.

EXAMPLE 1 An aqueous bath is prepared containing 114.1 grams per liter of cadmium chloride (CdCl -2 /z H O), 112.4 grams per liter of ammonium chloride and 180.5 grams per liter of EDTA. The pH of the bath is adjusted by the addition of ammonium hydroxide to a value of 4.9, and it is clear and free from suspended matter. The bath is divided into two portions, into each of which is immersed an evacuated, hollow cathode constructed of 1010 carbon steel and having an internal pressure of about 10" torr. In one portion, a bath temperature of about 43.5 centigrade is maintained and plating is carried out for 7 minutes 12 seconds at a current density of 50 amperes per square foot. In the other portion, the temperature is maintained at about 49 centigrade and plating is effected for a period of 14 minutes, 24 seconds at a current density of 25 amperes per square foot. Purified cadmium anodes are employed in each instance.

At the end of the plating operations, the vacuum probes are removed from the baths and placed in an oven and heated. Utilizing electronic equipment specifically designed for the purpose, the maximum pressure which occurs in the probe during heating is recorded and these values, along with the known hydrogen diffusion rate constant for the metal of the probe, are used to determine the diffusion of hydrogen through the metal. This, in turn, indicates the amount of hydrogen absorption, which reflects the degree of hydrogen embrittlement resulting from the plating operation.

The techniques employed for determining hydrogen diffusion may be better understood by reference to articles by G. T. Sink. The method is referred to in A Low Embrittlement Cadmium Plating Process published in the May 1968 issue of Plating magazine, and also in an article entitled Hydrogen Absorption by Steel During Cleaning presented on May 7, 1968 to the Seventh International Metal Finishing Conference at Hanover, Germany. This article is published by the Douglas Aircraft Company, Inc., as their paper No. 4849, dated May 1968. The equipment and the use thereof are also described in a number of United States patents to Samuel C. Lawrence, Jr., i.e., Nos. 3,241,056; 3,257,841; 3,258,- 683 and 3,357,907.

Utilizing a value of 30,000 as a measure of the maximum tolerable level of hydrogen transmission through the steel, which is based upon accepted industrial standards, it is seen that in both instances the baths of this example produce results which are below the maximum, and which are thus better than the standard set. In the first bath, the value reflecting hydrogen embrittlement is 29,500, whereas in the second bath this value is 24,900. The reported values are a factor of the heat peak pressure within the vacuum tube (i.e., the maximum pressure recorded during the heating procedure) times the known diffusion rate constant for hydrogen through the particular metal of the probe.

Although a comparison of the results attained in the two portions of the bath indicates that a 15 percent increase in hydrogen permeability is encountered by decreasing current density and increasing the temperature, this level of increase would result from the temperature change alone. Accordingly, the comparative data indicate that the embrittlement characteristics of the bath employed are quite independent of the current density, and this is most surprising because the prior art baths of this sort invariably produce a considerable increase in hydrogen difiusion (and embrittlement) as a function of decreasing current density.

EXAMPLE 2 An aqueous bath is prepared by dissolving 64 grams per liter of cadmium oxide, 118 grams per liter of nitrilotriacetic acid and 156 grams per liter of ammonium chloride in water. This bath is operated at a pH of about 5.5,

at a temperature of about 35 centigrade. Utilizing a.

cadmium anode and a current density of about 40 amperes per square foot, a sheet of low carbon steel is plated. Upon inspection of the coating, it is found to be adherent, ductile and nonporous, and it has a smooth, fine-grain appearance.

EXAMPLE 3 About 32 grams per liter of cadmium oxide, about 90.5 grams per liter of ethylenediaminetetraacetic acid and about 112 grams per liter of ammonium chloride are dissolved in water to produce a plating bath. One portion of this bath is used to plate a steel article at a pH of about 4.0, and another portion is used to plate another steel article at a pH of about 7.0. In each instance the temperature of the bath is maintained at about 50 centigrade and the current density applied is 50 amperes per square foot. On both articles adherent ductile coatings are produced which are smoothand fine-grained in appearance.

EXAMPLE 4 An aqueous plating bath is prepared by dissolving in water 128 grams per liter of cadmium oxide, 360 grams per liter of ethylene diaminetetraacetic acid and 150 grams per liter of ammonium chloride, and adjusting the pH to 0.5 with ammonium hydroxide. The bath is operated at a current density of about amperes per square foot and a temperature of about 60 centigrade using a fairly complexly configured cast iron article as the plating substrate. A strongly adherent and uniform coating having an attractive, fine-grained appearance is deposited upon all the surface of the article exposed in the bath, demonstrating the good throwing power of the bath.

Thus it can be seen that the present invention provides a novel and highly effective bath for producing adherent, ductile cadmium deposits on various metallic substrates. The bath is operable with high current efliciencies over a fairly wide range of current density and affords excellent throwing power and operability at low voltages. Cast and malleable irons may be electroplated with cadmium by utilization of the present baths, and certain steel alloys which are normally susceptible to embrittlement by hydrogen when plated in prior art baths may be plated with the baths described herein with tolerable levels of hydrogen absorption. The present invention also provides a novel and highly effective method whereby such desirable coatings can be deposited upon various substrates.

Having thus described the invention, we claim:

1. An aqueous electroplating bath for depositing cadmium, consisting essentially of water as the solvent, about 0.15 to 1.1 gram moles of cadmium ion per liter, about 1.0 to 4.0 gram moles of ammonium chloride per liter and, per mole of cadmium ion, about 0.8 to 1.6 moles of an aminopolyacetic acid chelating agent selected from the group consisting of ethylenediaminetetraacetic acid, diethylenetriamine-pentaacetic acid, nitrolotriacetic acid, and mixtures thereof, said bath having a pH of about 3.7 to 7.5.

2. The bath of claim 1 wherein the concentration of cadminum ion is about 0.3 to 0.7 gram mole per liter, the concentration of ammonium chloride is about 2.0 to 3.0 gram moles per liter, the molar ratio of said chelating agent to cadmium ion is about 1.0 to 1.25 and the pH is about 4.5 to 5.5.

3. The bath of claim 1 wherein said chelating agent is ethylenediaminetetraacetic acid.

4. In a method of electroplating cadmium, the steps comprising:

(a) preparing an aqueous bath consisting essentially of water as the solvent, about 0.15 to 1.1 gram moles of cadmium ion per liter, about 1.0 to 4.0 gram moles of ammonium chloride per liter and, per mole of cadmium ion, about 0.8 to 1.6 moles of an amino polyacetic acid chelating agent selected from a group consisting of ethylenediaminetetraacetic acid, diethylenetriamine-pentaacetic acid, nitrilotriacetic acid and mixtures thereof, said bath having a pH of 3.7 to 7.5;

(b) maintaining said bath at a temperature of about 25 to centigrade;

(c) immersing a workpiece having a metallic surface and a cadmium anode in said bath; and

(d) applying a voltage across said anode and workpiece to deposit a layer of substantially pure cadmium of said metallic surface.

5. The method of claim 4 wherein the concentration of cadminum ion is about 0.4 to 0.7 gram mole per liter, the concentration of ammonium chloride is about 2.0 to 3.0 gram moles per liter, and wherein said bath contains, per mole of cadmium ion, about 1.0 to 1.25 moles of said chelating agent.

6. The method of claim 5 wherein said bath has a pH of about 4.5 to 5.5 and is maintained at a temperature of about 32" to 52 centigrade, and wherein said applied voltage provides a current density of about 5.0 to 70.0 amperes per square foot.

7. The method claim 4 wherein said bath has a pH of about 4.5 to 5.5 and is maintained at a temperature of about 32 to 52 centigrade, and wherein said applied voltage provides a current density of about 5.0 to 70.0 amperes per square foot.

8. The method of claim 4 wherein said workpiece is comprised of a low carbon steel susceptible to hydrogen embrittlement.

9. The method of claim 4 wherein said chelating agent is ethylenediaminetetraacetic acid.

References Cited UNITED STATES PATENTS 2,090,049 8/1937 Hull 204-50 FOREIGN PATENTS 565,427 10/1958 Canada 20443 GERALD L. KAPLAN, Primary Examiner 

